Polarimetry ( Mr. Shaise Jacob )


The term Polarimetry may be defined as the study of the rotation of polarised light by transparent substance.

Optical rotatory power is useful for »

Qualitative analysis

Quantitative analysis

Elucidation of chemical structures

An ordinary light usually vibrates in all the planes → UNPOLARISED LIGHT

The vibrations may all be restricted to one direction only, in the perpendicular plane → PLANE POLARIZED LIGHT

Unpolarised light gets converted into a plane-polarized light by simply passing it through a lens called a NICOL Prism. (after William Nicol – the inventor)

NICOL Prism – Iceland Spar

                             Calcite ( CaCO3 form )

                             (or) Polaroid

A plane polarized light which consists of two components of fixed magnitude rotating in opposite directions to one another;

The right circulatory polarized light

The left circulatory polarized light

Plane polarized light is the vector sum of these two components


An optically active substance is one that rotates the plane of polarized light

(1) Lactic acid

Latin : Dexter = right

              designated by ‘d’

Latin : Laevus = left

              designated by ‘l’


   When a certain organic liquids, solutions (sugar) or quartz crystals are placed in the path of plane polarized light, the plane of polarization is rotated

Magnitude of rotation depend upon

Nature of the substance

Length of liquid column

Conc. Of the solution

Nature of the solvent

Temperature of the solution

WL of the light used

The rotatory power of a given solution is generally expressed as Specific rotation

a – observed angle of rotation

L – length in decimeters

C –  grams of substance in 100ml of solution


   Optical activity may be detected & measured by an instrument → Polarimeter

Light source – Sodium vapor lamp

Analyzer – another Nicol prism aligned to intercept the linearly polarized ray as it emerges from the sample solution

Determination of optical activity of Pharmaceutical substances

Optical rotation

Specific optical rotation

noting the angle through which the plane of polarization is rotated.



Determination of O.A compounds

Quantitative – sugar industry (sucrose)

O.A is the only one parameter, for distinguishing b/w D & L isomeric forms


» important practical application of polarimetry

» determination of high conc. of sugars

» visual saccharimeters called POLAROSCOPES

Source: http://www.slideshare.net/shaisejacob/polarimetry

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